Delayed type IV allergy to an antigen
Pathophysiology
Type IV hypersensitivity (T cell) reaction to external antigen e.g. nickel, chemicals, topical therapies, plants
Routes of exposure - direct skin contact, airborne contact, injection
Sensation stage:
generation of memory T cells following exposure to antigen (via Langerhans cells in epidermis and MHC-II)
Allergic stage:
activation of sensitised Th cells in response to antigen, causing release of inflammatory cytokines and cell-mediated cytotoxicity
Clinical features
Presents as an itchy, eczematous rash (vesicles, fissures, erythema), typically 24-48 hours after exposure
The rash may extend beyond the boundaries of immediate contact
This is unlike irritant contact dermatitis, where the skin changes are localised directly to the area of exposure
The patient is sensitised to the allergen over time, so they may have never had a problem in the past with the material until their presentation
Investigations
Patch testing - allergens prepared onto Finn chambers which are applied on the back and removed after 48 hours, readings at 48 and 96 hours
Management
Upon identification of the allergen, strict avoidance is necessary to prevent a recurrence
Symptomatic management includes emollients, oral antihistamines, topical hydrocortisone, and cool water soaks